Although non-A, non-B hepatitis agents cannot be detected by serologic means, they can be experimentally transmitted to chimpanzees and marmosets. These species have been useful in determining the infectivity titers of various non-A, non-B virus containing plasmas. Although most plasmas contain only 100-1000 infectious units per ml, one plasma was found to contain over 2 million infectious units per ml. This plasma has provided an inoculum suitable for characterization of the agent. We have recently demonstrated that at least one non-A, non-B agent contains essential lipids, a characteristic that will be important in the classification and, probably, the control of non-A, non-B hepatitis.